The arithmetic mean, also called the average, of a series of quantities is obtained by finding the sum of the quantities and dividing it by the number of quantities. In the series 1, 3, 5, 18, 19, 20, 25, the mean or average is 13—in other words, 91 divided by 7.
The median of a series is that point which so divides it that half the quantities are on one side, half on the other. In the above series, the median is 18.
The median often better expresses the common-run, since it is not, as is the mean, affected by an excessively high or low figure. In the series 1, 3, 4, 7, 55, the median of 4 is a truer expression of the common-run than is the mean of 14.
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